The invention relates to a method of measuring the oscillation frequency of a fluid jet in a fluidic oscillator using a temperature sensor whose temperature-dependent resistance varies as a function of the oscillation frequency of the jet.
Fluidic oscillators are well known to the person skilled in the art, and a particularly advantageous application thereof lies in the field of measuring a fluid flow rate, as described in document EP 0 882 951. A fluidic oscillator flowmeter 1, as shown in an exploded view in FIG. 1, comprises a flowmeter body made up of two side portions 2, 3 on either side of a central block 4 which includes a measurement unit 5.
On the inside, the side portion 2 defines a xe2x80x9cupstreamxe2x80x9d chamber (not shown) into which the fluid penetrates after passing through an inlet opening (not shown).
This chamber has a wall (not shown) in which the admission opening is formed. This wall acts as a deflector wall and it receives the impact of the fluid flow coming from the opening, splitting the flow and deflecting it towards two orifices (only one orifice 6 is visible in FIG. 1) formed in the deflection wall. Two passages 7, 8 extend the orifices respectively and direct the fluid flow fractions so as to make them converge on the inlet 9 of the measurement unit 5. As a general rule, this inlet is in the form of an elongate slot.
The measurement unit 5 has an oscillation chamber 10 in which an obstacle 11 is positioned as shown by two arrows in FIG. 1 so as to face the inlet 9.
The fluid penetrating into the oscillation chamber and encountering the front portion of the obstacle 11 oscillates transversely relative to the direction A in a plane parallel to the wall 12 and flows alternately past one end and the other end of said obstacle so as to leave the oscillation chamber via the outlet 13 in the direction A. The inlet 9 and the outlet 13 of the oscillation chamber are in alignment on the direction A. The oscillation chamber 10 is defined firstly by the wall 12 and secondly by another wall 15 parallel to said wall 12.
The flowmeter is installed between two pipes via flange couplings included in the admission opening (not shown) and the exhaust opening 14.
The fluid which leaves the oscillation chamber takes the passage 16 which forms a bend so as to guide the fluid towards the exhaust opening 14.
The side portion 2 is made integrally with the central block 4 of the flowmeter body which includes the measurement unit 5. The other side portion 3 is manufactured separately and is subsequently fitted onto the central block so as to act as a cover.
A cavity 17 is formed in the wall 15 so as to allow at least one flow sensor to be installed therein, e.g. a temperature sensor whose function is to detect the oscillations of the fluid in the oscillation chamber 10. Thus, because of the existence of a proportionality relationship between the oscillation frequency of the fluid jet in the oscillation chamber and the flow rate of the fluid along the pipe, the value of the fluid flow rate can be deduced by measuring the oscillation frequency. The fluid jet sweeps over the temperature sensor, thereby modifying its temperature by heat exchange, and consequently measuring temperature variations of the sensor by measuring variations in its electrical resistance makes its possible to determine the oscillation frequency of the jet.
The flowmeter preferably has two temperature sensors placed in the cavity 17 between the obstacle 11 and the inlet 9, symmetrically on either side of the inlet 9.
A duct 18 pierced by a hole is provided in the body of the flowmeter to pass electrical connections between said body (and in particular its sensor(s)), and a counter for informing the user of parameters such as total consumption of fluid and fluid flow rate.
Respective heat exchanges between the temperature sensor and both the fluid and the body of the flowmeter are shown in FIG. 2.
The heat exchanges between the sensor and the oscillating fluid are described by the following equation:
"PHgr"1=K1(Tthxe2x88x92Tg)
where:
"PHgr"1 is the power dissipated towards the oscillating fluid;
K1 is the thermal conductance between the sensor and the fluid;
Tth is the temperature of the sensor; and
Tg is the temperature of the oscillating fluid.
It should be observed that the thermal conductance K1 depends on the exchange area of the sensor, and on the thermal conductivity, the density, and the speed of the fluid. Consequently, when the fluid jet is oscillating, K1 depends on time.
Heat exchanges between the sensor and the body of the fluidic oscillator via the structure of the sensor and its packaging are described by the following equation:
"PHgr"2=K2(Tthxe2x88x92Tb)
where:
"PHgr"2 is the power dissipated towards the body of the fluidic oscillator;
K2 is the thermal conductance between the sensor and the body of the fluidic oscillator;
Kth is the temperature of the sensor; and
Tb is the temperature of the body of the fluidic oscillator.
It should be observed that the thermal conductance K2 is independent of fluid oscillations and consequently provides no information useful in determining the oscillation frequency.
The principle of conservation of energy leads to the following equation for describing the heat exchanges:             C      ⁢                        ⅆ                      T                          t              ⁢                              xe2x80x83                            ⁢              h                                                ⅆ          t                      +                            K          1                ⁢                  (          t          )                    ⁢              (                              T                          t              ⁢                              xe2x80x83                            ⁢              h                                -                      T            g                          )              +                  K        2            ⁢              (                              T                          t              ⁢                              xe2x80x83                            ⁢              h                                -                      T            b                          )              =      p    ⁢          (      t      )      
where:
C is the heat capacity of the sensor; and
p(t) is the instantaneous electrical power dissipated by the sensor.
It has been observed that for a sensor powered with DC, as is the case in prior art fluidic oscillator flowmeters, and under well-specified temperature conditions corresponding to the following:                     T        g            -              T        b              =                  V        2                    4        ⁢                  K          2                ⁢                  R                      t            ⁢                          xe2x80x83                        ⁢            h                                ,
where:
Rth is the electrical resistance of the sensor, information concerning the angular frequency xcfx890 of the oscillating fluid jet is lost because xcex4Tth(t)=0.
Such temperature conditions can be encountered, for example, when a hot fluid penetrates into a cold flowmeter. FIG. 3 shows how the amplitude of the signal Vm measured across the output terminals of the temperature sensor varies as a function of time t. It can be seen that under the above-specified temperature conditions, the signal diminishes until it reaches a value where it is no longer possible to measure the oscillation frequency of the jet.
An object of the present invention is to propose a method of measuring the oscillation frequency of a fluid jet in a fluidic oscillator using a temperature sensor, which method is insensitive to temperature conditions, and more particularly is insensitive to transient stages in which, for example, a hot fluid penetrates into a cold flowmeter.
According to the invention, this object is achieved by a method of measuring the oscillation frequency of a fluid jet in a fluidic oscillator using a temperature sensor whose resistance varies as a function of the oscillation frequency f0 of the jet, said method consisting in:
feeding the temperature sensor with an AC voltage of frequency f; and
determining the frequency components around the frequency 3xc3x97f in the signal output by the temperature sensor in order to determine the oscillation frequency f0 of the jet.
In a preferred implementation of the method of the invention, the frequency components in the output signal from the temperature sensor are determined by using the following steps:
measuring the resulting measurement signal V(t) across the terminals of the temperature sensor;
synchronously demodulating the measurement signal at the frequency 3xc3x97f; and
determining the frequency of the demodulated measurement signal, which frequency corresponds to the oscillation frequency f0 of the jet.
The step of synchronously demodulating the measurement signal at the frequency 3xc3x97f is preferably performed by multiplying the measurement signal V(t) by cos(3xc3x972xcfx80xc3x97fxc3x97t).